Hollow-core photonic crystal fibers are designed to guide optical beams via Bragg reflection at the core/cladding interface along the structure, as opposed to conventional fibers that guide light via total internal reflection along the waveguide. This property enables the fiber to guide light down its “core,” whose refractive index can be less than that of the cladding, even air or vacuum. Owing to the guided-wave nature of these fibers, the hollow core of the fiber can be filled with a variety of low or high refractive-index states of matter, including gases, cryogenic liquids (LN2), or critical-point media (ethane, Freon 113). By using such fibers, long interaction lengths can be realized that can far exceed lengths typical of free-space, focused media. This enables nonlinear optical interactions to be observed for laser powers not previously possible in short-interaction-length geometries.
This disclosure addresses PCMs via stimulated Raman, Brillouin or thermal scattering (SRS, SBS, STS respectively), externally seeded SBS or Brillouin-enhanced four-wave mixing (BEFWM). Since more than one spatial mode is required for wavefront-reversal of a general beam, a multi-mode HC-PCF will be required. In addition, a polarization-preserving HC-PCF is desirable as it will enable lower-threshold, high-fidelity wavefront reversal.
SBS conjugators have a rich history, being the first “self-pumped,” all-optical, passive wavefront compensator applied to solid-state lasers, MOPAs and energy-scalable architectures. See B. Ya. Zel'dovich, N. F. Pilipetsky, and V. V. Shkunov, Principles of Phase Conjugation, Springer Ser. Opt. Sci. 42, T. Tamir, Ed., Berlin, 1985 and D. M. Pepper, “Nonlinear Optical Phase Conjugation,” in The Laser Handbook, Vol. 4, M. L. Stitch and M. Bass, Editors (North Holland Publishing Company, The Netherlands, 1985), SBS PCMs are both simple and elegant: they typically use high-pressure gases with only a single input beam focused into the cell—the very beam whose wavefront-reversed replica is sought. The “time-reversed” beam is generated over the same focal volume as the input beam. Gases have been the most attractive SBS medium, and have proven themselves average-power scalable where the aberrations are small enough and the peak power high enough so that simple free-space focusing in a cell achieves threshold. They have thus found application to short-pulse, high-peak-power lasers. Low thresholds for highly aberrant beams, suitable for CW HELs, can be realized using SBS if guiding structures are used. Until now, however, suitable guiding structures did not exist for low-index media such as gases.
Using HC-PCF technology, new classes of simple and elegant PCMs may be realized using materials not previously employed. The emergence of hollow-core photonic crystal fiber (HC-PCF) enables one to realize PCMs with materials that can be configured into long interaction lengths. Such a device can service new wavelength regions (uv through the IR) as well as energy and power-scalable lasers (such as weapon-class lasers). In addition, now-classic SBS-based PCM configurations can now be made practical for a variety of applications, including de-polarization compensation and novel SBS-based power-scaling and output couplers as well as low-power applications owing to the long interaction lengths and confined (high-intensity) modes possible using HC-PCF. Prior to the development of the guided-wave structure (the HC-PCF), PCMs using waveguide geometries were constrained to materials with a refractive index greater than that of the cladding material of the guide (typically glass), thus limiting such devices to glass cores and a very limited number of high-index liquids whose reflective index is greater than that of glass (nglass≈1.5) lightpipes, such as CS2-filled capillaries. These nonlinear media have several limitations including competing nonlinear interactions, such as self-focusing, photochemical degradation, Raman scattering, and optical breakdown. Gases typically have higher thresholds for these competing effects, and, Xe (a promising candidate), in particular, has no Raman effect (it is an atom, not a molecule), with extremely high breakdown threshold.
Another feature of this disclosure is that it addresses high-power scaling of lasers. Traditionally, high-peak-power, yet, low average power, pulsed lasers are best suited for NLO interactions to be practical, owing to the higher order NLO optical processes (e.g., SBS). Currently, “loop” PCMs are being studied in the high-average power regime, but they have thermal, FOV, and fidelity limitations. The present approach circumvents these limitations and enables efficient NLO (SBS) PCMs to be realized using continuous-wave lasers with low-peak power, yet, scalable to high average powers, thereby opening up new classes of devices to service myriad applications.
Presently, PCMs provide a solution to realizing scalable, diffraction-limited lasers. The prior art includes a variety of phase-conjugate techniques for scaling of lasers, including SBS and photorefractive based devices. The SBS interaction works best with high-peak power lasers, owing to the constrained (limited) interaction lengths in free-space gas cells. SBS in solids and liquids suffer from optical damage, photochemical degradation, and competing nonlinear optical effects, such as optical breakdown, filamentation, SRS, self-phase modulation and self-focusing. On the other hand, the photorefractive materials suffer from optical damage (catastrophic, depoling) at high intensities and thermal effects at high optical absorption. However, SBS PCMs (the PCM of choice) function best under operating conditions of high-peak power and low-average power. To realize various classes of DEWs, a scalable laser is required to function at high-average powers, in the long pulse (quasi-cw) and cw regimes, which is counter to operational parameters preferred for SBS devices. Recently, so-called “loop” PCMs have been employed by HRL and Raytheon with limited success at the required laser powers, but suffer from limited wavefront-reversal fidelity, resulting in a loss of brightness. Our approach shows great promise to fill the gap, and, thus, provide a mitigation candidate for this important element in a DEW system.
Such a laser can also enable one to realize a source for manufacturing applications, including welding and drilling. The “loop” PCMs use either thermal nonlinearities as real-time holographic gratings at a loop vertex, as well as optical gain elements as the nonlinear medium. The loop devices have design constraints owing to the thermal nonlinearity, which can limit the phase-conjugate fidelity and FOV, especially as one scales to high-average powers. The optical gain media are promising, but, may also have limitations at high-average powers, since operation is constrained to intensities on the order of the saturation intensity of the medium (where, presumably, the induced gratings have the greatest phase-conjugate Bragg selectivity). In addition, being an active material, additional prime power is required to optically invert the energy-level populations. Finally, being an oscillator, the loop geometries are much more complex than passive PCMs (e.g., this invention) and, possess potential pulsation effects, which can cause optical damage at high-powers, as well as potentially damage the materials that are processed by the output power, such as laser welders, etc.
FIG. 1 is a schematic diagram of a representative Phase Conjugate Master Oscillator Power Amplifier (PC-MOPA) schematic which represents an application of a high peak power, high-energy scaled laser system using a series of laser amplifiers 1–4 easing using external excitation E. A PCM 5 is employed to result in a diffraction-limited high brightness system output. The use of a HC-PCF as the PCM 5, as disclosed herein, can enable this prior art system to also function as a high-average power (quasi-CW) system for material processing and DEWs. The prior art was limited to very short pulses so that high peak powers were obtainable which is required by prior art PCMs. That is, prior art systems were limited by the devices, whereas the present invention can enable more applications and systems to be realized.
This invention is not obvious given the prior art of which the inventor is aware. The prior art does, indeed, discuss how to realize PCMs using SBS. However, in all these cases where gas is used as the nonlinear medium, free-space focusing is described, since, “conventional wisdom” compels one to reason that guided-wave structures require guiding materials with refractive indices greater than the cladding (confining) layers in an optical waveguide. Researchers have, in the past, employed glass cores with in-diffusion of selected gases, such as H2, as a means to realize Raman scattering over long interaction lengths, but, such an approach was not very practical and remained an intellectual curiosity for the past 2 decades. SBS was, indeed, observed using liquid-filled guides, filled with high-index media, such as CS2. Again, the paradigm here was to limit the guiding material to candidates with a refractive index greater than the glass guide (nCS2≈1.62>nglass≈1.5). Therefore, the community would not have considered guiding light in a gas, or a cryogenic liquid or critical point media, owing to the fact that its index (≈1) is far less than the cladding glass layer(s).
The emergence of HC-PCF opens the door to realizing optical guiding in media whose index is less than that of the cladding. In fact, several papers have been published that describe Raman scattering in single-mode HC-PCFs, filled with H2. Also, a paper by F. Benabid, J. C. Knight, G. Antonopoulos, and P. St. J. Russell, “Stimulated Raman Scattering in Hydrogen-Filled Hollow-Core Photonic Crystal Fiber”, Science 298, 399 (2002) discusses employing a Xe-filled, single-mode HC-PCF to demonstrate high-peak power pulsed laser propagation over long lengths; the goal of this system is to confine very short pulses of high peak power over very long distances for long-haul communication purposes, without pulse spreading, etc. The optimized system was designed using a single-mode HC-PCF, with a gas fill, which would minimize any and all nonlinear optical effects, which can limit the system bandwidth. The Xe gas fill was selected for several reasons to optimize the communication application of the HC-PCF. For example, the presence of Xe enables very high peak powers to be confined with minimal nonlinear effects (since Xe does not have a Raman effect and, moreover, Xe has a much smaller nonlinear index effect relative to air). The operating conditions, as well as the pressure of the Xe gas were also chosen so that gIL>10 to avoid SBS, which would be unwelcome in their communication application, where g is the SBS gain, I is the optical intensity and L is length. For long-haul communication links, the technical community also specifies the use of single-mode fibers, since multi-mode fibers can limit the communication bandwidth due to modal dispersion. Hence, a multimodal fiber would be deleterious in such an application, and therefore, designers of long haul communication systems specifically avoid that class of fiber.
Now, one may suppose that, given these efforts, the extension to multi-mode HC-PCFs using SBS would be obvious. However, in phone discussions (by the first named inventor) with several experts in this technology area, where the topic of multi-mode HC-PCFs was mentioned (not in regard to this disclosure, but, in the general context of modal dispersion), none of the researchers ever considered the topic of multi-mode propagation in these structures, much less polarization effects again since the goal of the technical community involves long-haul fiber communication links. Given that these two effects (even in the linear regime) are very general, and, moreover, that they represent critical technological issues that are necessary to understand in arriving at this invention (not to mention SBS, etc.), it appears that the technical community has yet to consider the notions discussed in this disclosure.